Gas-over-oil actuators (also called gas/hydraulic actuators or gas powered actuators) are typically installed on valves, such as isolating valves, in a natural gas distribution pipeline and used to control the valves with high-pressure natural gas. For example, the gas-over-oil actuator may be powered by natural gas pressure taken directly from the natural gas pipeline, e.g., 75 bar. In this conventional system, all electrical equipment must be certified for the specific hazardous area, as does any electrical connection between a control room and the gas-over-oil actuator.
In one example, and referring now to FIGS. 1A and 1B, a conventional gas-over-oil actuator 10 includes an open side gas hydraulic tank 12, and a closed side gas hydraulic tank 14. A mounting bracket 16 may be used to install the gas-over-oil actuator 10 on a valve disposed in a natural gas pipeline (not shown).
As depicted in FIG. 1B, the gas-over-oil actuator 10 further includes an opening solenoid valve 18 and a closing solenoid valve 20, both of which may be installed on an actuator control, for example. A command, such as an open/close command, from wired powered inputs, e.g., two different 24 VDC, are sent to the opening solenoid valve 18 and the closing solenoid valve 20 to control and operate the valve. In addition, a limit switch 22 (FIG. 1A) is installed on top of the gas-over oil-actuator 10 and is used for an open/closed position feedback. Two different wired signals are used as open/closed position feedback.
In addition, two different manual override systems, the oil override system (not shown) and the solenoid valve manual override system (not shown), are typically installed on the gas over-oil actuator 10. The oil override system moves the valve if the pipeline is empty or if the natural gas pressure is insufficient. The solenoid valve manual override system moves the valve if the electrical equipment or power supply fails.
The gas-over-oil actuator 10 can be remotely operated by local electric pushbuttons or from a main dispatching center. In one example, one or more lights on a control panel (HSV) may depict the open/closed valve position, and the electrical control panel (HSV) is located in a safe area. In some plant layout configurations, two pressure transmitters are also installed in the pipeline, one upstream and the other downstream the valve. The pressure transmitters monitor the gas pressure in the pipeline and verify any gas leakage once the valve is in a closed position, for example.
In the foregoing conventional system, all the instruments installed in the plant are physically wired with armored cables and cable glands, for example, to the control panel installed in the control room. Due to inherent porosity of cable insulation layers, permeation phenomena has been experienced. In addition, all electric wiring is arranged by implementing suitable trays along the cable paths to prevent mechanical stress.